As a first conventional art of a motor control device, for example, a usage called web handling control for continuously conveying a band-like material (web) by using a roll rotated by a motor as in the case of a steel rolling line is envisaged. In the conventional art for the usage described above, for the purpose of preventing imbalance of a load torque in motors connected by a conveyed material from being increased, a speed control unit has been provided with a function called drooping for a long time (for example, see Patent Literature 1).
In the drooping control, a value corresponding to a proportional multiple of a signal indicative of a motor current, that is, a motor torque, is subtracted from a speed command, thereby providing a drooping characteristic (drooping) in which a motor speed decreases when the motor torque increases. In this manner, a function of preventing a load current of a specific motor among the plurality of motors connected by the conveyed material from being excessive is realized.
A motor control device described in Patent Literature 1 includes, as illustrated in FIG. 1 of Patent Literature 1, a speed control unit which is subjected to conventional drooping control (specifically, a speed control unit provided with a drooping characteristic by subtracting a drooping amount corresponding to a proportional multiple of an output of the speed control unit from the speed command) inside the motor control device. Further, the motor control device described in Patent Literature 1 additionally includes an acceleration/deceleration current computing unit for computing a torque required for acceleration/deceleration of the motor so that an output of the speed control unit and an output of the acceleration/deceleration current computing unit are added to be output as a torque command to the motor.
In this manner, a change in drooping amount in the case where the motor speed is accelerated/decelerated in response to a change in speed command can be eliminated. Thus, even when the motor speed is accelerated/decelerated, the motor can follow the command with high accuracy.
As a second conventional art of the motor control device, a machine tool controlled by an NC control device or the like, which is a usage different from that of the first conventional art, is envisaged. In the conventional art for the above-mentioned usage, parallel driving control in which a single driven object (work piece) is driven while being synchronously controlled by two motors is performed (for example, see Patent Literature 2).
In the case of the parallel driving control described above, the following problem occurs due to accuracy of a position detector for each of the motors or mechanical accuracy when the driven object and the motors are mechanically coupled to each other. Specifically, when the position of each of the motors is made completely equal to the command, a mechanical torsion occurs in the driven object to generate forces of the motors which pull each other (hereinafter, referred to as an interaxial interference force). As a result, heat generation or electric degradation of the motors or mechanical degradation of the driven object becomes a problem.
To cope with the above-mentioned problem, in the second conventional art, position control is performed on each of the motors while the same position command is issued to the two motors. At the same time, in the second conventional art, torque commands output by the motor control devices respectively to the two motors or actual torque detection values are compared with each other. Then, the position command to one of the motors is corrected based on a signal obtained by a difference between the torque commands or the actual torque detection values. In this manner, the interaxial interference force is suppressed to further suppress the heat generation or the electrical degradation of the motors or the mechanical degradation of the driven object.